In the production of hermetic enclosures, such as packages for semiconductor devices, a housing or base member containing a semiconductor die or chip is typically sealed with a lid which is usually attached to the base by means of a peripheral charge of solder, and the package is sealed by assembling the lid adjacent to the base and by subjecting this assemblage to heat in order to melt the solder. For many applications a pre-formed peripheral charge of solder (known as a solder "preform") is pre-attached to the cover during the manufacturing process, so that the lid and solder are provided together, as a combined unit. Such combined hermetic sealing covers for a package for a semiconductor device, a method for their fabrication and a method for fabricating a sealed package with them, are described, for example, in U.S. Pat. Nos. 3,874,549, 3,946,190 and 3,823,468.
Typically, the cover or lid comprises a substrate that is made of plastic or of metal (e.g., iron-nickel or iron-nickel-cobalt alloys such as KOVAR (KOVAR is a registered trademark of CRS Holding, Inc.)), which is sometimes coated with a thin layer of either nickel or gold or with alternating layers of nickel and gold. One conventional technique for applying these coatings to metallic covers is electroplating, and plated metallic covers as well as methods of fabricating and plating such covers are described in U.S. Pat. Nos. 4,243,729, 4,284,481, 4,601,958 and 4,666,796.
For some applications the body of the lid either is constructed entirely of glass, or it is constructed of plastic or metal but with a central opening adapted to accept a glass insert, in either case thereby allowing electromagnetic radiation such as light to reach the electronic device even after the package is sealed. Many of the physical characteristics of the cover, such as the size, shape, composition, configuration and thickness of the lid itself, as well as the composition and thickness of the coating layers (if any), and the size, shape, composition and thickness of the glass insert (if any), are typically specified within narrow tolerances by the manufacturer of the electronic device that is intended to be housed within the package.
During the process of sealing a semiconductor package, residual atmospheric gases can sometimes remain inside the package, due to incomplete evacuation. Also, after the package is sealed and the electronic device within the assembled package is placed into use, the normal operation of the electronic device often generates harmful gases and/or moisture which also become trapped inside the package. Although the prior art has attempted to deal with the problem of excess moisture trapped within the package, by suggesting the inclusion of powder or granular dessicant materials within the package, or by teaching the formation of gels or pastes comprising dispersions of such dessicant materials and the subsequent application thereof to the interior of the package, the prior art has not addressed the reduction or elimination of common but deleterious atmospheric gases, such as oxygen, carbon monoxide, carbon dioxide and ammonia, as well as nitrates and sulfates, from the interior or cavity of the package.
It is generally well known that certain elemental metals, specifically, silicon, germanium and titanium, can provide strong adsorption sites for water, water vapor and common atmospheric gases. However, in general, highly pure layers of these materials cannot be electroplated onto the outer surface of a typical coated hermetic sealing cover for a semiconductor package. Moreover, these three elemental metals differ somewhat from one another in the nature and extent of their chemical and physical adsorption properties, and they also differ from one another with respect to the layer thickness and grain size at which their adsorbent properties can be optimized. However, none of the techniques (e.g., stencil printing) suggested by the prior art for applying a moisture-adsorbent material to the interior of a semiconductor package allows sufficient control over either the thickness or the grain size of the material being applied to enable the effective use of any of these more efficient, adsorbent elemental metals in a semiconductor package.
It is therefore the principal object of this invention to provide a sealing cover or an insert for a semiconductor package, methods for making such a cover and/or insert and methods for making such a package, in which a material that is capable of adsorbing both moisture and other common atmospheric gases is selectively coated on the sealed side of the cover or on the insert utilizing a technique which allows precise control of the thickness and grain size of the adsorbent material being applied, and thus allowing those parameters to be adjusted depending upon the specific nature of the adsorbent material being used and the other requirements of the manufacturer of the electronic device.